"Nato is deeply concerned by threats to the physical safety of Estonian diplomatic staff, including the ambassador, in Moscow, as well as intimidation at the Estonian embassy," the statement said.

On Wednesday, Estonia's foreign ministry said there was an attempt to physically assault Ms Kaljurand at a news conference, as members of "Nashi" tried to disrupt it.

It said the incident amounted to a violation of diplomatic conventions.

Reports said Russian police also scuffled with activists outside the Estonian embassy, arresting one person as protesters attempted to prevent diplomats entering or leaving the building.

Following the disturbances, the European Union said it would send a delegation to raise concerns with Russia over the increasing violence.

US State Department spokesman Tom Casey called on the Moscow authorities to do everything they could to reduce tensions.

More than a quarter of Estonia's 1.3 million people are ethnically Russian, and speak Russian. However, half of them do not have Estonian citizenship.

During the years of Soviet occupation after the war tens of thousands of Estonians were killed. They say their country was effectively colonised, with many Russians being brought in as workers and military personnel.